1. Field of the Invention
This invention relates to an infrared radiation sensor. More specifically, it relates to an infrared radiation sensor for the noncontacting determination of a temperature of a subject under test, particularly a temperature of a tympanic membrana of an organism.
2. Description of the Prior Art
Heretofore, the practice of effecting transfer of infrared radiation between a temperature sensing part serving as a temperature sensor and a subject under test and determining the temperature of the subject based on a resultant change in temperature of this temperature sensing part has been in vogue. Various proposals aimed at miniaturizing such heat sensitive infrared radiation sensors and imparting exalted sensitivity thereon have been introduced to the art.
The infrared-radiation which can be received is very feeble particularly when the temperature of a given subject is near or below normal room temperature as in the case of the temperature of an organism. In adopting a thermistor type temperature sensing means, therefore, it is necessary to devise how to enlarge the thermistor constant and, at the same time, decrease the thermal capacity of the sensor, and prevent to the fullest possible extent the energy of the received radiation from dispersing.
For the purpose of ensuring acquisition of the latter effect, an infrared radiation sensor of a construction wherein a temperature sensing part is formed on a minute bridge structure part so as to decrease as much as possible the amount of the thermal energy to be lost from the temperature sensing part and the temperature sensing part is provided with a means for airtightly sealing the periphery thereof in a vacuum so as to repress to the fullest possible extent the transfer of heat to the ambient air has been proposed.
The means for sensing infrared radiation utilizes such a principle as that the electric resistance of a film such as of amorphous germanium, amorphous silicon, polycrystalline germanium, or polycrystalline silicon is varied with a temperature and, therefore, fits for miniaturization of a sensor. When this means is adopted, however, the highest thermistor constant (B constant) that is attainable at all is only 3500 K. and the sensitivity, therefore, is not fully satisfactory.
As a means for enhancing the sensitivity relative to an infrared radiation, an infrared radiation sensor using a pn junction diode has been disclosed in JP-A-05-142,039 and in literature (Technical Digest of 9th Sensor Symposium, 1990; pp. 71-74).
This sensor utilizes the temperature characteristics which are obtained by the pn junction diode when this diode is biassed in the forward direction.Of this pn junction diode, the temperature coefficient of forward voltage is -1.6 mV/.degree.C. and the sensitivity is 33 V/W at most. Thus, the diode is still deficient in sensitivity.
In addition, a Schottky junction temperature sensor utilizing the phenomenon that the saturated current which flows in the reverse direction when a voltage is applied to a Schottky junction diode in the reverse direction is largely varied with a temperature has been disclosed (JP-A-05-40,064).
The Schottky junction temperature sensor of this nature has a high equivalent thermistor constant of about 7600 K. This fact suggests the possibility of this sensor acquiring more than twice as high sensitivity as the conventional thermistor type infrared radiation sensor.
The Schottky junction diode is encased in a single crystal silicon chip and is not furnished with any special measure for thermal insulation. It, therefore, entails the problem that it has a too large thermal capacity to be effectively heated by feeble infrared radiation.
When polycrystalline or amorphous silicon is used as the material for the Schottky junction diode, this Schottky junction diode still falls short of perfectly forming a highly sensitive infrared radiation sensor because the leak current apparently originating in a crystal defect is large and the change of the current due to a change in temperature is proportionately small.
As a means for forming a single crystal silicon faithfully in a designed shape on a bridge structure part, a method for forming a floating single crystal thin film has been disclosed (JP-A-04-186,833). This method makes use of the p+ etch stop technique which resides in diffusing boron (B) at a high concentration in silicon for allowing single crystal silicon to remain on the bridge structure part. This technique necessitates provision of an etch stop protecting area by encircling an n type silicon part forming an element with a p type silicon. The provision of the etch stop protecting area renders difficult the formation of a bridge structure part with a minute thin film.
An object of this invention, therefore, is to provide a novel infrared radiation sensor.
Another object of this invention is to provide an infrared radiation sensor of high accuracy suitable for a thermometer to be used for the noncontacting determination of a temperature of a subject under test, particularly a temperature of a tympanic membrana of an organism, by utilizing the temperature characteristics of the saturated current of the Schottky junction diode in the reverse direction.
Yet another object of this invention is to provide an infrared radiation sensor suitable for a thermometer to be used for the noncontacting determination of a temperature of a subject under test by utilizing a temperature sensor, particularly a Schottky barrier thermistor, formed in an n type single crystal area of a semiconductor supported on a minute bridge structure part formed of a thermally and electrically insulating thin film.